When to visit Sorrento & the Amalfi Coast?

It is perhaps best to consider your reasons for visiting Sorrento. There are loads of places to visit and it has a very well organised tourism infrastructure and transport system. English is also widely spoken and (accepting the limited beaches) it is family friendly.

Since the tourism is driven largely from northern Europe (visitors from North America have declined in line with the dollar) it doesn't have the dramatic increase in visitors to the coast in the Italian favourite; August (Ferragosta). However if you are serious about visiting many of the sites (e.g. Pompeii) then the cooler Spring will make this a far more comfortable experience. Accommodation with a pool may be a consideration for some; Sorrento itself has limited beach areas and they can get very busy in July/August.

Fortunately to get to Pompeii, Herculaneum and, if you wish, Naples you can use the excellent Circumvesuviana train service, the traffic in this direction from Sorrento can be chaotic and makes driving slow. Likewise the ferries, hydrofoils and "Metro Del Mare" operate services from the beginning or end of April until the beginning or end of October.